[Boatanchors] SB-220 with parasitics

[KA2PTE]

When I bought my Johnson Thunderbolt amp back to life, was told to try 
gettering the tubes
(EIMAC 4-400A's) first before anything else. Best way is to build a fixture 
for the tube and do it on the bench
with some low plate voltage according to some youtube videos on the subject.

I opted to skip it, and the tubes turned out to be ok. I also didnt really 
put the amp
into full duty very fast, I babied it with low RF drive during some initial 
tests and always had the screen
voltage present with the tubes mostly in cutoff bias.

Curious if that amp has a variable neutralizing cap? Mine was dirty , and I 
think it was
encouraging parasitics here and there. Once I cleaned the dirt from the 
electrodes,
most all my problems were resolved.

On a side note, the way the cap was designed is interesting. One electrode 
was like a bell housing,
the other was directly in the center, and when the arc happend, it actually 
rang like a bell. At first I thought it
came from the 400A tube and it was the glass ringing from the arc.


--
Steve Hearns [ KA2PTE ]




----- Original Message ----- 
From: "JAMES D COMSTOCK" <comnoz2 at comcast.net>
To: "Keith Densmore" <ve3gem at gmail.com>; <boatanchors at mailman.qth.net>
Sent: Friday, December 24, 2021 12:25 PM
Subject: Re: [Boatanchors] SB-220 with parasitics


>I would guess the "parasitic bang" would have been a flashover. This is 
>usually caused by gas in the tube. The gas can come from overheating the 
>elements which boils the gas from the metal or more commonly with a 3-500, 
>a tiny leak of air into the tube. The flashover does clear some of the gas 
>buildup. It may or may not have damaged the tube beyond use. The balance of 
>the gas buildup can be cleared by gettering. The getter is on the plate of 
>that tube and is not active until the plate is beginning to glow red hot. 
>Gettering without causing a flashover is done by maintaining high current 
>at low plate voltages for some time. But if flashovers continue to happen 
>intermittently  after it has been gettered then there is likely air slowly 
>entering the tube. That was a common problem with EIMAC tubes with latter 
>build dates. Jim
>
>
>
>> On 12/24/2021 9:23 AM Keith Densmore <ve3gem at gmail.com> wrote:
>>
>>
>> Safe Holidays to Everyone,
>> I have agreed to look at a Heath SB-220 amp for a friend. The amp, tubes
>> look like new but one of the 3-500's has had a parasitic bang, at least
>> once and maybe twice. (On the same tube.)
>> I cannot find much info on the web but the question is, can such a tube 
>> be
>> reformed? Run the filament for some hours? Power it up with lower plate
>> and/or filament voltage? Any hope or is it dead?
>> If anyone has experience in this area, your advice is welcome.
>>
>> Thanks,
>> Keith ve3gem
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